refactor(backend): migrate db.DB to db.Conn PoolProxy across all handlers

Replace direct *pgxpool.Pool usage with PoolProxy wrapper across the entire backend:

- db.DB renamed to db.Conn (*pgxpool.Pool -> *PoolProxy)
- JWT functions now accept context.Context instead of using context.Background()
- Handler DB calls route through PoolProxy for per-test transaction support
- Fixture/helper/testdb functions accept Querier interface for decoupling
- Query ordering fixed in bookings handlers: COUNT after data query to avoid pgx conn busy
- Time truncation fixed: time.Date instead of Truncate(24*time.Hour) for week start calc
- testmain_test.go files updated with SeedBaseline and NewPoolProxy

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/internal/square/square_dev_test.go

@@ -81,7 +81,7 @@ func TestDevClient_CreateCheckout_PendingThenCompleted(t *testing.T) {
 		if err == nil && completed.Status == "COMPLETED" {
 			break
 		}
-		time.Sleep(5 * time.Millisecond)
+		time.Sleep(200 * time.Millisecond)
 	}
 	if err != nil {
 		t.Fatalf("GetCheckout failed: %v", err)
@@ -131,7 +131,7 @@ func TestDevClient_CreateCheckout_NoTip(t *testing.T) {
 		if err == nil && completed.Status == "COMPLETED" {
 			break
 		}
-		time.Sleep(5 * time.Millisecond)
+		time.Sleep(200 * time.Millisecond)
 	}
 	if err != nil {
 		t.Fatalf("GetCheckout failed: %v", err)